AN ICBM (INTERMEDIATE COMPLEXITY BOX MODEL) FOR INVESTIGATING DEEP TIME BIOGEOCHEMICAL CYCLES

Changes in the redox state of the ocean-atmosphere system have important consequences for the coupled biogeochemical cycles of C, N, P, O, and S. Under less oxidizing conditions than present, denitrification and sulfate reduction in the oceanic water column may have been widespread. While previous work suggests that P can be recycled more efficiently under these conditions, the implications for the N cycle are less clear. To investigate the coupled behavior of CNPOS under a wide range of conditions we have developed an intermediate complexity box model (ICBM) that uses process-based equations for the CNPOS system in a multi-box representation of ocean processes. The model includes gyres, upwelling regions and a shelf, and has multiple reservoirs with a depth resolution from 10 to 400 meters. It includes simple plankton population dynamics and explicit representation of N-fixation, nitrification, denitrification, and anammox. The model has been tested against data sets from both the modern open ocean and Black Sea. The model retains the conceptual simplicity of box models but has sufficient depth resolution to produce realistic global fluxes and depth profiles. We deliberately chose this format for paleoceanographic applications in deep time, when boundary conditions on ocean circulation necessary for GCM and related models are poorly constrained.

We are investigating several questions about possible responses of ocean biogeochemical cycles to redox changes. 1) for approximately modern rates of deep water ventilation, at what pO2 do deep water dysoxic or anoxic conditions become widespread? 2) what is the effect of hypsometry on ocean biogeochemical cycles? 3) With reduced oxygenation denitrification rates generally increase. Under what conditions will this impose N limitation more generally on surface water NPP? A model can only meaningfully investigate these questions if it can successfully couple the CNPOS system, as there are strong feedbacks among the major cycles. Preliminary results suggest that pO2 must remain >50% PAL to avoid widespread bottom water anoxia, providing a constraint that can be tested against observation in the Paleozoic and Mesozoic sedimentary records. High rates of denitrification can be compensated by high rates of N fixation only if there are few limitations on the growth of N fixers.